1. Field of the Invention
The present invention relates to a brushless DC motor and, more particularly, to a brushless DC motor having a hydrodynamic bearing system which eliminates whirl instability.
2. Description of the Related Art
The brushless DC motor, or spindle motor, has been commonly used in various applications wherein a precise rotating movement is required. These applications include a laser scanner and a disk drive motor. The laser scanner includes a mirror rotatable about an axis to deflect a laser beam for the projection of image rays to an exposure station to expose the surface of a moving photoreceptor. The scanner requires a high response speed to drive signals and precise angular positioning of the mirror.
The conventional brushless DC motor, however, has been found to exhibit large amplitude vibration under certain operating conditions. The motor typically includes a stator comprising a core having windings arranged thereabout, a rotor having spaced magnets positioned thereon and a shaft supporting the rotor at one end thereof. Bearings support the shaft in the radial and axial directions, the bearings being lubricated by a fluid.
Large amplitude vibration can be caused by imbalance, shaft flexibility, bearing flexibility, fluid film forces in the bearings as the shaft rotates, etc. One particularly common vibration mode occurs at approximately half the shaft rotation frequency. It has further been shown that half frequency vibration occurs when the radial load is small.
The related art has disclosed devices which attempt to reduce the problems associated with vibrational instability in such motors.
U.S. Pat. No. 4,657,803 to Von der Heide et al discloses an electric motor with a substantially cylindrical air gap between a stator and a rotor, the stator being fitted to a bearing support. The stator is connected to the bearing support by means of an elastic damper, the stator and bearing support being separated from one another by an air gap.
U.S. Pat. No. 4,135,119 to Brosens discloses a limited rotation motor having a stator and a rotor mounted for limited rotation relative to the stator. The stability of a radial rotor position in its support bearing is enhanced by an imbalance in the bias flux magnetic circuits which continually produce a resultant radial force on the rotor so that a well defined radial position of the rotor is maintained over the range of limited rotation.
U.S. Pat. No. 4,851,731 to Saotome et al discloses a brushless DC motor in which vibrations of a rotor axle are prevented without increasing manufacturing costs by providing the magnetic field between a stator and a rotor to be asymmetrical so that the magnetic attracting force of the rotor to the stator is also asymmetrical. The asymmetrical force causes the axle of the rotor to be urged in the same radial direction relative to an axle bearing in the stator, thus eliminating movements of the axle in radial directions. In a first embodiment, the magnitude of lateral forces exerted on the axle of the rotor can be varied in accordance with the height and area of a stepped portion. A second embodiment has an upper surface which is inclined at an angle with respect to rotor axle and bearing. A third embodiment has an inclined bearing which will incline axle at an angle with respect to the vertical to ensure that the axle will always be against one side of the bearing.
Other attempts at stabilizing rotational vibration in brushless DC motors include preloading the journal bearing on the shaft and applying a fixed direction radial load. Preloading may be accomplished in several different ways. For example, precise patterns can be formed on the surface of the journal or bearing, or the journal surface or bearing can be made out-of-round. The manufacturing tolerances in pattern formation are usually in the 0.001 inch range and very expensive to produce. Likewise, it is expensive to make the journal surface or bearing out-of-round. The application of a fixed direction radial load is shown in FIG. 1. Heretofore, this load application was performed by providing a stationary magnet, manufacturing different elements (see the above-mentioned U.S. Pat. No. 4,851,731), etc. The cost of the motor was therefore increased, additional installation time was required and the number of components required was increased.
In fluid-filled bearings, clearance is provided between a rotor shaft and bearing. it is difficult to avoid shaft vibrations relative to the bearing caused by this clearance. As wear occurs between the shaft and bearing, the clearance increases and, therefore, so does vibration. If a fluid is not used, either expensive ballbearings which burn, provide unacceptable noise, and wobble must be used or excessive wear caused by friction will result.
The related art thus attempts to achieve the elimination of vibration instability in a brushless DC motor by manufacturing in a more complicated manner or by the addition of parts.